Thermoelectric properties, metal-insulator transition, and magnetism: Revisiting the Ni1−xCuxS2 system

The $\mathrm{N}{\mathrm{i}}_{1\ensuremath{-}x}\mathrm{C}{\mathrm{u}}_{x}{\mathrm{S}}_{2}$ pyrite ($x\ensuremath{\le}0.07$) is a model material where the impact of electron filling on a half-filled ${e}_{\mathrm{g}}$ band system can be separated from changes in the bandwidth, since the unit cell volume does not change with $x$. This contrasts with the $\mathrm{Ni}{\mathrm{S}}_{2\ensuremath{-}x}\mathrm{S}{\mathrm{e}}_{x}$ system where the size difference between ${\mathrm{S}}^{2\ensuremath{-}}$ and $\mathrm{S}{\mathrm{e}}^{2\ensuremath{-}}$ makes the bandwidth vary at constant band filling. Our magnetic measurements show that there exists a clear relation between the presence of the antiferromagnetic phase developing below ${T}_{\mathrm{N}2}=29.75\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ and charge localization. With the addition of Cu in $\mathrm{N}{\mathrm{i}}_{1\ensuremath{-}x}\mathrm{C}{\mathrm{u}}_{x}{\mathrm{S}}_{2}$, the ground state evolves towards a metallic paramagnetic state. These findings are discussed with the scenario based on charge ordering at low temperature in the charge transfer insulator $\mathrm{Ni}{\mathrm{S}}_{2}$ pyrite, and also considering possible conducting surface states at low temperatures. At 300 K, the thermal conductivity decreases by 2.5 as $x$ increases from 0.00 to 0.07, even though the latter is much more electrically conductive than the former. This considerably extends the range of values for the thermal conductivity of the $\mathrm{M}{\mathrm{S}}_{2}$ pyrites, from the largest in $\mathrm{Fe}{\mathrm{S}}_{2}$ to the lowest in the present $\mathrm{N}{\mathrm{i}}_{1\ensuremath{-}x}\mathrm{C}{\mathrm{u}}_{x}{\mathrm{S}}_{2}$ samples.